The present invention relates to plasma processing apparatus and methods, and more particularly to plasma processing apparatus and methods that use a controlled potential plasma source in order to minimize particulate formation within the plasma.
In recent years, plasma processing has emerged as one of the most versatile and efficient techniques for the processing of materials in several of the largest manufacturing industries in the world. For example, in the electronics industry, plasma-based processes are indispensable for the manufacture of very large-scale integrated (VLSI) microelectronic circuits (or chips). Plasma processing is also a critical technology in the aerospace, automotive, steel, biomedical, flat-panel displays, solar cells, and toxic waste management industries. For an overview of the many and varied applications that rely on plasma processing for materials processing, see, e.g., PLASMA PROCESSING OF MATERIALS, Scientific Opportunities and Technological Challenges, National Research Council (National Academy Press, Washington, D.C. 1991).
In general, plasma processing involves the creation and maintaining of a plasma, and the application of the plasma to a particular material that is to be processed by the plasma. A plasma is a partially or fully ionized gas containing electrons, ions, and neutral atoms and/or molecules. In a typical plasma processing application, the nonlinear collective interactions of the electrically charged particles with each other, with neutral atoms and molecules, and with electric and magnetic fields, are used to selectively process a particular material that is exposed to the plasma. For example, in a plasma deposition or etching application, the plasma is used to selectively process a semiconductor wafer on which VLSI microelectronic circuits are being formed.
In plasma deposition, and many other plasma processing applications, one of the technological challenges that must be addressed is the control of xe2x80x9cparticlesxe2x80x9d and xe2x80x9cparticulatesxe2x80x9d in the plasma. (Note, as used herein, the terms xe2x80x9cparticle xe2x80x9d and xe2x80x9cparticulatexe2x80x9d are used as synonyms.) A xe2x80x9cparticlexe2x80x9d is generally considered as a small piece of material that is larger than a cluster of a few molecules, but small enough to remain suspended in a fluid for a time. A xe2x80x9cparticlexe2x80x9d may originate from a source external to the plasma, or may be formed within the plasma due to the physical and chemical processes occurring within the plasma.
While the presence of certain types of particles may be advantageous to some kinds of plasma processing operations, e.g., because the particles help promote a desired chemical or physical process carried out during the plasma processing operation, most particles are not advantageous. When a particle or particulate is not advantageous to the plasma process it is referred to as a xe2x80x9ccontaminantxe2x80x9d. Dust particles are an example of contaminants that interfere with the delicate plasma etching operation used in making VLSI chips. See, e.g., Donovan, Particle Control For Semiconductor Manufacturing (Marcel Dekker, Inc. New York 1990). The presence of a dust particle having a size less than about 1 xcexcm (where one xcexcm is 10xe2x88x926 meters), for example, renders most VLSI processing impossible, where circuit traces and other component sizes and spacings on the VLSI chip may only be on the order of 0.35-1.0 xcexcm. Hence, contaminants as small as 0.1 xcexcm may present a problem with the precise deposition and/or etching that must be achieved in most VLSI processing applications. Thus, there is a critical need in the plasma processing art for a way to remove and/or control the presence and/or location of contaminants in the plasma so that such contaminants do not interfere with the plasma processing operation being performed.
Several techniques are known in the art for removing contaminants from the plasma that originate from sources external to the plasma. Generally, such techniques, e.g., filtering the gases used to form the plasma, have proven effective at reducing the density of such contaminants to manageable levels. A significant need still persists, however, for eliminating or minimizing the presence of internally-formed contaminants, i.e., contaminants that originate from particles formed within the plasma itself due to the physical and chemical processes occurring within the plasma. The present invention is directed to this need.
To conceptually understand how contaminants are formed within a plasma, reference is made to FIG. 1A, which schematically illustrates a plasma-formation device 20. As seen in FIG. 1A, the plasma formation device includes opposing electrodes 22 and 24, each of which is connected to a voltage potential source 26. By introduction of appropriate gases into the region between the electrodes, and by application of an appropriate potential to the electrodes, a plasma 28 is formed. Such plasma 28 is made up of electrons and ions. At the same time that the ions and electrons in the plasma are spent, e.g., as they are attracted to and strike the electrodes and/or interact with other elements and/or a workpiece being processed by the plasma, new electrons and ions are also formed. Hence, the generation of the plasma, and the loss of the plasma, is a continual process. As this continual process proceeds, the plasma, as a whole, is maintained at a potential that is positive with respect to the chamber wherein the plasma is confined, yet the plasma itself tends to stay electrically neutral. To remain electrically neutral, the electron flux leaving the plasma must generally be equal to the positive ion flux leaving the plasma. Because electrons are smaller and lighter than ions, and have a significantly higher thermal velocity than do ions, the electrons tend to diffuse out of the plasma much faster than the heavier and slower ions. However, to achieve a charge balance within the plasma, an ambipolar electric field EA is created that tends to also drag the positive ions out of the plasma, as well as to retard the escape of electrons from the plasma. This causes the plasma potential to be positive relative to the vessel or chamber wherein the plasma is formed. Because the plasma potential is positive relative to the vessel or chamber wherein the plasma is formed, negative ions tend to get trapped within the plasma. Such negative ions thereafter serve as nucleation points for dust particles and other contaminants within the plasma.
Further, it is noted that any particulates in the plasma are also charged negatively. As the particulate grows from the nucleation site, due to its presence in the plasma, it begins to acquire a large negative charge. The plasma charges up the particulate negatively because there is an initial large net negative flux to the particle. This occurs because the electrons, being lighter, have a larger thermal velocity than the positively charged ions. As the particle charges up, it begins to repel the electron current and the charging process is slowed. The charging ceases in equilibrium when the negative current and positive current to the particle exactly cancel. What is left, though, in equilibrium is a large negative charge on the particle. This will occur even if the nucleation site is a neutral gas molecule, atom, or a positive ion. Thus, no matter what the initial nucleation site may be, the particulates quickly become trapped within the plasma. Once trapped, further particle growth occurs due to agglomeration and the particle can become large enough to cause damage to the workpiece.
Further, when negatively-charged particulates are present in the plasma, such particulates limit the rate at which certain plasma deposition processes can be carried out. There is thus a need in the art for a plasma source that removes such negatively-charged particulates so that the plasma deposition and other processes can be carried out at a more rapid rate.
The trapping of negative ions, and subsequent formation of contaminants within a plasma, has been demonstrated, both through modeling and experimentation. See, Hollenstein, et al., xe2x80x9cDiagnostics of Particle Genesis and Growth in RF Silane Plasmas by Ion Mass Spectrometry and Light Scattering,xe2x80x9d NATO Advanced Research Workshop on Formation, Transport and Consequences of Particles in Plasmas, Chateau de Bonas, France, Aug. 30-Sep. 3, 1993. Further, once contaminants have been formed within the plasma, e.g., from negative ions that have been trapped within the plasma, various techniques have been proposed for moving them out of the plasma. See, e.g., Selwyn, Gary S., xe2x80x9cPlasma particulate contamination control. I. Transport and process effects,xe2x80x9d J. Vac. Sci. Technol. B, Vol. 9, No. 6, pp. 3487-92 (1991); Selwyn et al., xe2x80x9cPlasma particulate contamination control. II. Self-cleaning tool design,xe2x80x9d J. Vac. Sci. Technol. A, Vol. 10, No. 4, pp. 1053-59 (July/August 1992). Such techniques thus address a problemxe2x80x94how to deal with negatively-charged particulates/contaminants in the plasmaxe2x80x94that exists because negative ions and negatively-charged particulates have become trapped in the plasma. A better approach to solving the overall contaminant problem would be to prevent negative ions from ever becoming trapped in the plasma in the first place, as well as to remove negatively-charged particulates from the plasma in their early formation stages. The present invention takes this latter approach.
The present invention addresses the above and other needs and solves the internally-formed contaminant problem within a plasma by preventing negative ions and negatively-charged particulates from ever becoming trapped in the plasma. Such trapping is prevented through the use of special plasma-formation apparatus that uses a plasma chamber having a unique electrode and electrode-biasing configuration that advantageously controls the plasma potential. Such configuration maintains the plasma formed in the chamber positive with respect to a first set of electrodes placed at opposite ends of the chamber, yet allows the plasma to be negative with respect to a second set of electrodes placed along opposing sides of the chamber. A magnetic field, having magnetic field lines perpendicular to the faces of the first set of electrodes, is generated using a pair of Helmholtz coils placed around the chamber. The magnetic field strength is carefully controlled to be sufficiently weak to allow negatively- and positively-charged ions, or negatively-charged particulates, in the plasma to escape across the magnetic field lines to the second set of electrodes, yet sufficiently strong to prevent electrons from escaping to the second set of electrodes. That is, electron flow in the plasma is restricted by the magnetic field to flow along the magnetic field lines, but ion-flow and particulate flow or movement is not so restricted.
The restriction of electron flow to flow along the magnetic field lines, in combination with the higher thermal velocity of the electrons (which allows electron current to dominate ion current in the plasma), causes the plasma to respond to the potential applied to the first set of electrodes much more than it does to the potential applied to the second set of electrodes. As a result, the plasma potential is effectively controlled by the potential applied to the first set of electrodes. This allows the plasma potential to be controlled by application of an appropriate potential to the first set of electrodes so that the plasma is maintained slightly positive with respect to the first set electrodes (as is required in any plasma formation device), yet at the same time allows the potential of the plasma to be controllable with respect to the potential applied to the second set of electrodes. In particular, the plasma may be made negative with respect to the second set of electrodes so that any negative ions will be lost from the plasma to the second set of electrodes before such ions are able to serve as nucleation points for contaminants.
Advantageously, with the plasma being made negative with respect to the second set of electrodes, not only are negative ions lost from the plasma, but any negatively-charged particulates that may be in the plasma (from whatever source) are also drawn out of the plasma. Such negatively-charged particulates will typically be very small, i.e., in their early formation stages (and thus not too much larger than many negative ions), when they are drawn out of the plasma, thereby preventing the growth of such particulates to sizes that could be detrimental to the particular plasma processing application being employed.
The plasma-formation apparatus of the present invention thus includes a plasma formation chamber within which two pairs of opposing electrodes are placed. Opposing faces of one pair of the electrodes is substantially orthogonal to the opposing faces of the other pair of electrodes. The first pair of electrodes is generally referred to as the control electrodes, with one end electrode being placed at each end of the plasma formation chamber. A longitudinal axis aligns the two control electrodes with each other. The other pair of electrodes is referred to as the reference electrodes, with one reference electrode being placed along opposing sides of the plasma formation chamber. A lateral axis aligns the two reference electrodes with each other. Each reference electrode is connected to a known electric potential. Each control electrode is also connected to an electric potential having a dc component. The dc bias on the control electrodes is negative with respect to that of the reference electrodes. The plasma discharge can be powered by applying an alternating radio frequency (rf) field on the control electrodes. Alternatively, the plasma discharge can be powered by placing the rf field on the reference electrodes. However powered, the resulting plasma has a potential close to the negative dc bias placed on the control electrodes, and is thus negative with respect to the reference electrodes. In this way, any negative ions in the plasma are attracted to the reference electrodes. A uniform magnetic field (B-field) is created by a pair of Helmholtz coils that surround the plasma formation chamber. The B-field has magnetic field lines perpendicular to the faces of the control electrodes, i.e., parallel with the longitudinal axis that aligns the control electrodes with each other. The B-field is sufficiently strong to restrict the movement of electrons within the plasma to movement along the B-field lines, yet sufficiently weak to allow any negative ions in the plasma to be drawn laterally through the B-field lines to the reference electrodes.
It is thus a feature of the present invention to provide a plasma formation device wherein negative ions and negatively-charged particulates are not allowed to become trapped in the plasma, but are drawn out of the plasma as soon as they are formed.
It is a further feature of the invention to provide a plasma source wherein the plasma is not positive relative to all portions of the chamber within which the plasma is formed, but wherein at least one electrode is maintained at a potential that is slightly positive relative to the plasma (i.e., the plasma is negative relative to such electrode) so that any negative ions or negatively-charged particulates in the plasma are attracted to such electrode.
It is another feature of the invention to provide such a plasma source wherein the potential of the plasma (i.e., whether it is positive or negative relative to the chamber within which it is formed) can be controlled through application of an external bias potential to a control set of electrodes.
It is an additional feature of the invention to provide such a plasma source as above-described that can be used to control the plasma potential, and remove negative ions and negatively-charged particulates therefrom, regardless of how the plasma discharge is powered (rf, microwave, etc.)
It is another feature of the invention to provide a plasma formation device that minimizes the formation of internally-formed contaminants within the plasma.
It is yet an additional feature of the invention to provide a plasma source wherein the plasma potential is controlled by a bias potential applied to a first pair of opposing electrodes, and wherein the formation of internally-formed contaminants within the plasma is prevented, and negatively-charged particulates are removed, by drawing such negative ions and negatively-charged particulates out of the plasma to a second pair of opposing electrodes, which pair is maintained at a potential that is positive with respect to the plasma potential.
It is still a further feature of the invention to provide a plasma processing apparatus, and method of controlling the same, that produces xe2x80x9ccleanxe2x80x9d plasma (relatively contaminant free plasma) that can be used for a wide variety of applications, including VLSI processing, flat panel displays, and solar cells; and wherein such xe2x80x9ccleanxe2x80x9d plasma allows many of the plasma processing steps, e.g., plasma deposition, to be carried out at a more rapid rate than has previously been possible.